Method for heat treating components

ABSTRACT

A method for heat treating a superalloy component includes heating a superalloy component to a first temperature, cooling the superalloy from the first temperature to a second temperature at a first cooling rate in a furnace, and cooling the superalloy component from the second temperature to a final temperature at a second cooling rate. The second cooling rate is higher than the first cooling rate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of the U.S. patent application Ser. No.15/636,511 filed Jun. 28, 2017.

BACKGROUND

This disclosure relates to a method of heat treating components, and inparticular, components comprising heat treating powder metallurgyprocessed superalloys.

Powder metallurgy superalloys provide improved damage tolerance, creepresistance, and strength capability to various components, includingcomponents for gas turbine engines. The physical characteristics of thesuperalloy components depend on the microstructure of the components.The microstructure of the components is, in turn, partially dependent ona number of parameters selected during the heat treatment of thecomponents. Heat treatment typically includes one or more stages thatrequire moving the components between various equipment to performdifferent types of cooling processes. Furthermore, cooling rates of thecomponent during some process steps, such as solution and quenchingprocesses, are difficult to control, thereby leading to microstructuralvariations.

SUMMARY

A method for heat treating a superalloy component according to anexample of the present disclosure includes heating a superalloycomponent to a first temperature, cooling the superalloy from the firsttemperature to a second temperature at a first cooling rate in afurnace, and cooling the superalloy component from the secondtemperature to a final temperature at a second cooling rate. The secondcooling rate is higher than the first cooling rate.

In a further embodiment of any of the foregoing embodiments, the firstcooling step is performed at a first pressure, and the second coolingstep is performed at a second pressure higher than the first pressure.

In a further embodiment of any of the foregoing embodiments, the secondpressure is between about 1 and 20 bar (0.1 and 2 MPa).

In a further embodiment of any of the foregoing embodiments, the firsttemperature is above a solvus temperature for the superalloy componentand the second temperature is below the solvus temperature.

In a further embodiment of any of the foregoing embodiments, the furnaceincludes a fan operable to provide convection within the furnace, andthe fan has a first speed during the first cooling step and a secondspeed during the second cooling step. The second speed is higher thanthe first speed.

A further embodiment of any of the foregoing embodiments includesperforming the second cooling step immediately after the first coolingstep without removing the component from the furnace.

In a further embodiment of any of the foregoing embodiments, thesuperalloy component comprises a supersolvus processed powder metallurgysuperalloy. The average grain size is between about 20 to 120 μm (0.787to 4.72 mils) in diameter.

In a further embodiment of any of the foregoing embodiments, thesuperalloy component comprises a nickel-based superalloy.

In a further embodiment of any of the foregoing embodiments, the firstcooling rate causes formation of a γ′ phase of the nickel-basedsuperalloy at grain boundaries.

In a further embodiment of any of the foregoing embodiments, theformation of the γ′ phase at grain boundaries causes serration of thegrain boundaries.

A method for heat treating a superalloy component according to anexample of the present disclosure includes heating a superalloycomponent to a first temperature, cooling the superalloy from the firsttemperature to a second temperature at a first pressure in a furnace,and cooling the superalloy component from the second temperature to afinal temperature at second pressure. The second pressure is higher thanthe first pressure, without removing the superalloy component from thefurnace.

In a further embodiment of any of the foregoing embodiments, at leastone of the first and second pressures are provided by backfilling thefurnace with a gas.

In a further embodiment of any of the foregoing embodiments, the secondpressure is between 1 and 20 bar (0.1 and 2 MPa).

In a further embodiment of any of the foregoing embodiments, the furnaceincludes a fan operable to provide convection within the furnace, andthe fan has a first speed during the first cooling step and a secondspeed during the second cooling step. The second speed is higher thanthe first speed.

In a further embodiment of any of the foregoing embodiments, the firstcooling step has a first rate of cooling and the second cooling step hasa second rate of cooling. The second rate of cooling is greater than thefirst rate of cooling.

In a further embodiment of any of the foregoing embodiments, thesuperalloy component comprises a nickel-based superalloy. The firstcooling rate is selected to cause formation of a γ′ phase of thenickel-based superalloy at grain boundaries, which causes serration ofthe grain boundaries.

A system for heat-treating a superalloy component according to anexample of the present disclosure includes a furnace operable to cool asuperalloy component from a first temperature to a second temperature ata first cooling rate and to cool the superalloy component from thesecond temperature to a final temperature at a second cooling rate. Thesecond cooling rate is higher than the first cooling rate. The firsttemperature is above a solvus temperature for the superalloy componentand the second temperature is below the solvus temperature.

In a further embodiment of any of the foregoing embodiments, thesuperalloy component is cooled from the first temperature to the secondtemperature at a first pressure, and is cooled from the secondtemperature to the final temperature at a second pressure. The secondpressure is higher than the first pressure.

In a further embodiment of any of the foregoing embodiments, the secondpressure is between 1 and 20 bar (0.1 and 2 MPa).

In a further embodiment of any of the foregoing embodiments, the furnaceincludes a fan operable to provide convection within the furnace. Thesuperalloy component is cooled from the first temperature to the secondtemperature when the fan is operated at a first fan speed, and is cooledfrom the second temperature to the final temperature when the fan isoperated at a second fan speed. The second fan speed is higher than thefirst fan speed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows the microstructure of a superalloy component.

FIG. 2 shows a method of heat treating a superalloy component.

FIG. 3 shows a graph of the temperature of the superalloy component overtime.

FIG. 4 schematically shows a furnace for heat treating the superalloycomponent.

DETAILED DESCRIPTION

FIG. 1 is a schematic view of the microstructure of a superalloycomponent 20. In one example, the component 20 is a component for a gasturbine engine, such as a cover plate, retaining plate, side plate, heatshield, compressor or turbine rotor or disk, or another gas turbineengine component. However, it will be appreciated that this disclosureis not limited to gas turbine engine components. The superalloycomprises a powder metallurgy superalloy, such as a nickel-based powdermetallurgy superalloy. More particularly, the material is a coarse-grainprocessed powder metallurgy superalloy. Superalloys include crystallineregions, called grains 24. The grains 24 include various solid phases ofthe superalloy which form the microstructural matrix. In most cases,matrices form precipitates 26 to establish precipitate strengtheningmechanisms for capability enhancement. In nickel-base superalloys, oneparticular phase, known as the γ′ (gamma prime) phase, contributes tothe strength of the superalloy at elevated temperatures and to its creepresistance. Coarse-grain supersolvus processed powdered metallurgysuperalloys typically have average grain sizes between about 20 to 120μm diameter (0.787 to 4.72 mils). Example coarse-grain superalloys arePRM48, ME16, IN-100, ME501, ME3, LSHR, Alloy 10, RR1000, and NGD2.

The grains 24 are separated by grain boundaries 28. The grain boundaries28 in FIG. 1 are serrated, but other grain boundaries 28 can be smooth.A higher degree of serration of the grain boundaries 28 yields improveddamage tolerance of the component 20. Increasing the amount ofprecipitates 26 at the grain boundaries 28 increases the degree ofserration of the grain boundaries 24.

FIG. 2 shows a method 100 of heat treating a superalloy component. FIG.3 shows a graph of the temperature of the superalloy over time. In step102, a superalloy is heated above its solvus temperature T1 using anyknown ramp and soak method. The solvus temperature T1 depends on theparticular composition of the superalloy, but is generally a temperatureabove which one or more solid microstructural phase 26 either partiallyor completely dissolves into a parent matrix grain.

In step 104, the component 20 is cooled to a temperature T2 that isbelow the solvus temperature T1 over a time t1. This first cooling stepcauses solid precipitates 26, such as precipitates of the γ′ phasediscussed above, to precipitate into the superalloy matrix. The exacttemperature T2 and the time t1 depend on the particular composition ofthe superalloy and are selected to allow for desired amount ofprecipitates 26, in particular at grain boundaries 28, which results inserration at grain boundaries 28. This can be observed by metallographicanalysis of specimens extracted from fully heat treated components.

Step 104 is performed in a furnace 30, shown in FIG. 4. The furnace 30includes a high-powered heat exchanger 32 and a high-powered fan 34. Thefurnace also includes a controller 36 operable to control thetemperature of the furnace (i.e., operation of the heat exchanger 32)and the fan 34 speed, as well as pressure in the furnace. The controller36 includes the necessary hardware and/or software to control thefurnace 30 as described herein.

The furnace is held at a first pressure P1 during step 104 bybackfilling the furnace 30 with gas, such as helium, argon, or nitrogen,or another inert gas. In one example, the pressure P1 can be atmosphericpressure (approximately 1 bar) or higher. The fan 34 allows forconvective cooling within the furnace by circulating the gas. In oneexample, no convection is provided during step 104. That is, the fan isoff. In another example, convection is provided during step 104 byrotating the fan at a fan speed F1.

The furnace 30 allows for control of a cooling rate R1, which isdependent on the temperatures T1 and T2, pressure P1, time t1, fan speedF1, and type of gas. Control of the cooling rate R1 allows for controlover the amount of serration of the grain boundaries 28 in the component20, which in turn affects the physical properties of the superalloy asdiscussed above. This is in contrast to fluid quench cooling methods,which are difficult to control and can require part-specific insulatedcooling, modification of superalloy forging methods, and/orpart-specific cooling. Furthermore, the control over the cooling rate R1allows for greater control of microstructure of components 20 having awider variety of cross sections and sizes without sacrificing alloystrength. This means smaller parts and near-net forgings can bemanufactured without oversizing the parts, reducing manufacturing costsand lead times. Optimal temperature T1, pressure P1, time t1, fan speedF1, and type of gas vary with the composition of the superalloy, as themicrostructure formation and growth is compositionally dependent on thekinetics of the alloy system. This is broadly driving towards a targetintergranular precipitate size, which will contribute to the severity ofgrain boundary serration and is also alloy dependent, but intergranularprecipitate size may be approximately 500 nm (0.0197 mils) equivalentdiameter or greater.

In step 106, the component 20 is cooled from temperature T2 to a finaltemperature T3 from time t1 to a time t2 by gas quenching. Step 106allows for further refinement of the microstructure of the component 20.Step 106 is performed in the furnace 30 at a pressure P2 with the fanoperating at a fan speed F2. The cooling rate R2 depends on thetemperatures T2 and T3, pressure P2, time t2, fan speed F2, and type ofgas in the furnace 30. As above, these parameters vary with the specificcomposition of the superalloy.

Higher pressure and increased convection provided by the fan 34 improveheat transfer between air/gas in the furnace 30 and the component 20,which increases the rate of cooling. Both the pressure P2 and the fanspeed F2 during step 106 are higher than the pressure P1 and fan speedF1 during step 104, which provides a cooling rate R2 greater than thecooling rate R1. In one example, the ratio of the cooling rates R1 to R2is between about 2:1 and 10:1. In a further example, the differencebetween the pressures P1 and P2 is between about 2 Bar and 10 Bar andthe difference between the fan speeds F1 and F2 is between about 10% to100% of maximum capability of the fan. Higher cooling rates during step106 improve tensile strength and fatigue properties of the superalloy.As above, pressure P2 is achieved by backfilling the furnace with a gas.The pressure P2 is higher than atmospheric pressure. In a particularexample, P2 is between about 1 and 20 bar (0.1 and 2 MPa). In a furtherexample, P2 is between about 10 and 20 bar (1 and 2 MPa).

In one example, steps 104 and 106 are performed in immediate successionwithout removing the component 20 from the furnace 30. This eliminatesvariability induced by the need to transfer the component 20 betweenvarious pieces of equipment, such as fluid quenching equipment andfurnaces. Transferring the component 20 would introduce variability intothe cooling process and, in turn, into the microstructure of thecomponent 20. Furthermore, the controller 36 can be programmed tooperate the furnace 30 at a particular temperature, pressure, and fanspeed for a particular amount of time. This allows for automated controlover the temperature, pressure, and convection in the furnace 30 duringsteps 104 and 106, and automated transition between steps 104 and 106,which reduces process variability.

Furthermore, the foregoing description shall be interpreted asillustrative and not in any limiting sense. A worker of ordinary skillin the art would understand that certain modifications could come withinthe scope of this disclosure. For these reasons, the following claimsshould be studied to determine the true scope and content of thisdisclosure.

What is claimed is:
 1. A system for heat-treating a superalloycomponent, comprising: a furnace operable to cool a superalloy componentfrom a first temperature to a second temperature at a first cooling rateand to cool the superalloy component from the second temperature to afinal temperature at a second cooling rate, wherein the second coolingrate is higher than the first cooling rate, and wherein the firsttemperature is above a solvus temperature for the superalloy componentand the second temperature is below the solvus temperature.
 2. Thesystem of claim 17, wherein the superalloy component is cooled from thefirst temperature to the second temperature at a first pressure, and iscooled from the second temperature to the final temperature at a secondpressure, wherein the second pressure is higher than the first pressure.3. The system of claim 18, wherein the second pressure is between 1 and20 bar (0.1 and 2 MPa).
 4. The system of claim 17, wherein the furnaceincludes a fan operable to provide convection within the furnace,wherein the superalloy component is cooled from the first temperature tothe second temperature when the fan is operated at a first fan speed,and is cooled from the second temperature to the final temperature whenthe fan is operated at a second fan speed, wherein the second fan speedis higher than the first fan speed.